The oncotic pressure exerted by proteins in human plasma has a normal value of approximately 26 to 28 mm Hg. Although this pressure appears to be small in relation to osmotic pressure (28 mm Hg ≅ 1.4 mOsm/kg H2O), it is an important force involved in fluid movement across capillaries (see •Fig. 1.9 Relationshipbetweentheconcentrationofplasmapro-teinsinsolutionandtheosmoticpressure(oncoticpressure)theygenerate.Proteinconcentrationisexpressedingramsperdeciliter.Normalplasmaproteinconcentrationisindicated.Notehowtheactualpressuregeneratedexceedsthatpredictedbyvan’tHoff’slaw.80 60 40 20 0 2 6 10 14 Protein (g/dl) Predicted by van’t Hoff’s law Normal plasma ActualOsmotic pressure (g/dL)
The total concentration of all molecules in a solution can also be measured as specific gravity. Specific gravity is defined as the weight of a volume of solution divided by the weight of an equal volume of distilled water. Thus the specific gravity of distilled water is 1. Because biological fluids contain a number of different molecules, their specific gravities are greater than 1. For example, normal human plasma has a specific gravity in the range of 1.008 to 1.010.
Thespecificgravityofurineissometimesmeasuredinclinicalsettingsandusedtoassesstheurineconcentratingabilityofthekidneys.Thespecificgravityofurinevariesinproportiontoitsosmolality.However,becausespecificgravitydependsbothonthenumberofmoleculesandontheirweight,therelationshipbetweenspecificgravityandosmolalityisnotalwayspredictable.Forexample,inpatientswhohavereceivedaninjectionofradiocontrastdye(molecularweight> 500g/mole)forx-raystudies,valuesofurinespecificgravitycanbehigh(1.040to1.050),eventhoughtheurineosmolalityissimilartothatofplasma(e.g.,300mOsm/kgH2O).
The plasma membrane is a lipid bilayer composed of phospholipids and cholesterol, into which are embedded a wide range of proteins. One class of these membrane proteins (membrane transport proteins or transporters) is involved in the selective and regulated transport of molecules into and out of the cell. These transporters include water channels (aquaporins), ion channels, solute carriers, and ATP-dependent transporters.
The movement of molecules across the plasma membrane through ion channels and solute carriers is driven by chemical concentration gradients and, chemical concentration gradients and electrical potential differences (charged molecules only). The electrochemical gradient is used to quantitate this driving force. ATP-dependent transporters use the energy in ATP to transport molecules across the membrane and often establish the chemical
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and electrical gradients that then drive the transport of other molecules through channels and by the solute carriers. Water movement through aquaporins is driven by an osmotic pressure difference across the membrane.